A typical engine control system for a modern spark ignition internal combustion engine includes an electronic controller operable to measure engine operating conditions and operator inputs, and operable to control various systems and actuators based upon the measured conditions and inputs. The typical electronic controller includes an interface electrically connected to a plurality of engine and vehicle sensors via one or more wiring harnesses. The engine and vehicle sensors measure various engine operating conditions and operator demands. Measured conditions include intake air temperature, as well as other engine conditions, including for example, engine rotational speed and position, engine load, vehicle speed, engine coolant temperature, engine air/fuel ratio, accessory demands, and the operator's demand for power. The engine control system is operably connected to engine and powertrain actuators and systems that act to control the engine, in response to the engine operating conditions and operator demands. Typical actuators and systems include, for example, fuel injectors, fuel pump, idle air control valve, exhaust gas recirculation valve, throttle control valve, transmission solenoids, and an exhaust system.
A skilled practitioner designs and implements software algorithms and calibrations which are executed in the electronic controller to measure the engine operating conditions and operator demands, and control the engine actuators accordingly. In addition to software algorithms and calibrations for controlling engine actuators in response to operating conditions and operator demands, the controller also includes algorithms and calibrations that perform on-board monitoring and diagnosis of various components and systems. The software algorithms and calibrations are typically developed and inserted into software of the engine controller during engine development, prior to start of production.
The intent of the on-board monitoring and diagnosis system is to ensure that an operator is notified when performance of a component or system has degraded to a level that emissions performance has been substantially affected. The operator is notified via a malfunction indicator lamp of the system degradation, and a skilled mechanic may use a diagnostic scan tool to recover information from the controller that assists in diagnosing a malfunction and subsequently repairing the system.
Each of the engine control and diagnostic algorithms relies upon accurate reading of various engine operating conditions and operator inputs in order to properly control the various systems and actuators. An inaccurate reading of one or more of the operating conditions has the potential to adversely affect engine performance, driveability, or emissions, or cause a false failure of one or more engine or vehicle diagnostics.
A skilled practitioner uses intake air temperature information read from the intake air temperature sensing circuit as input to a substantial number of engine control and diagnostic algorithms. An intake air temperature reading may affect whether another algorithm is executed, e.g. if entrance criteria for executing the algorithm includes the intake air temperature reading must be within a specified range. Intake air temperature reading may affect execution of another algorithm, e.g. wherein the intake air temperature reading affects calculated mass of air flowing to the engine, thus affecting engine fueling. An improperly operating engine caused by an inaccurate intake air temperature reading may lead to increased engine-out and tailpipe emissions. Therefore, accurate reading of intake air temperature is required for proper engine operation on an engine equipped with an electronic control system.
Past diagnostic systems have required that the intake air temperature sensor be monitored to determine when the sensor or circuit fails. Standardized diagnostic trouble codes (‘DTC’) have been established as a result of federal and state On-Board Diagnostic regulations. They include DTCs for monitoring and reporting intake air temperature sensor malfunctions, as follows: DTC P0112: Intake Air Temperature (IAT) Sensor Circuit Low Voltage, and DTC P0113: Intake Air Temperature (IAT) Sensor Circuit High Voltage. As indicated, these DTCs are set when an on-board diagnostic algorithm detects that the Intake Air Temperature (IAT) Sensor Circuit is out of range because the monitored output voltage for the sensor is excessively low or excessively high. Passenger cars and trucks sold in the United States since model year 1996 have been required to have such diagnostic algorithms on-board.
The algorithms associated with detection of DTCs P0112 and P0113 are most effective in detecting failures caused by an open electrical circuit or a shorted electrical circuit. However, these diagnostic algorithms do not account for all intake air temperature sensing circuit failure modes encountered during vehicle operation. A typical intake air temperature monitoring circuit includes a sensing device, a wiring harness including one or more electrical connectors, a voltage supply, one or more electrical filtering circuits, and an analog-to-digital converter that provides interface to the controller. The sensing device is typically a resistive temperature device, wherein electrical resistance of the device changes in response to a change in temperature. The engine controller typically employs a voltage divider circuit to measure a voltage drop across the resistive temperature device, and is able to translate the measured voltage drop into a temperature reading using a predetermined calibration. A change or intrusion into the intake air temperature sensing circuit that affects the voltage drop measured by the controller may affect the intake air temperature reading. Such changes or intrusions may include, for example, corrosion at a connector leading to increased electrical resistance in the circuit, among others known to one familiar with electrical circuit design. Therefore, what is needed is a method and apparatus which monitors the intake air temperature sensing circuit and detects changes in measurements by the controller that lead to erroneous IAT readings.